Fixed: don't destroy dependent associations if we cannot destroy the Country

Previously :ensure_not_default  callback could stop deleting the
Country but associations (with dependent: :destroy) were
deleted whatsoever, as this is a rails bug (see
rails/rails#3458)

Author: damianlegawiec

core/app/models/spree/country.rb

@@ -3,6 +3,10 @@ class Country < Spree::Base
     has_many :states, dependent: :destroy
     has_many :addresses, dependent: :restrict_with_error
 
+    # we need to have this callback before any dependent: :destroy associations
+    # https://github.com/rails/rails/issues/3458
+    before_destroy :ensure_not_default
+
     has_many :zone_members,
              -> { where(zoneable_type: 'Spree::Country') },
              class_name: 'Spree::ZoneMember',
@@ -11,8 +15,6 @@ class Country < Spree::Base
 
     has_many :zones, through: :zone_members, class_name: 'Spree::Zone'
 
-    before_destroy :ensure_not_default
-
     validates :name, :iso_name, presence: true, uniqueness: { case_sensitive: false, allow_blank: true }
 
     def self.default